i. Angioplasty Procedures and Devices
In recent years, percutaneous transluminal angioplasty has become a commonly performed mode of treatment for various obstructive vascular disorders, including stenotic lesions of the coronary arteries, iliac arterial obstructions, femoral popliteal arterial obstructions, renal arterial obstructions, cerebrovascular arterial obstructions, coarctations of the aorta, stenotic arteries in transplant organs, stenotic saphenous vein bypass grafts, stenotic dialysis fistulas and stenotic portal systemic shunts.
To date, the most commonly used type of transluminal angioplasty procedure has been "balloon" angioplasty. Balloon angioplasty generally involves the positioning of a balloon catheter adjacent to or within the stenotic region of a blood vessel such that, upon inflation of the angioplasty balloon, the balloon will exert outward compressive force against the walls of the blood vessel thereby compressing the stenotic lesion and/or diametrically stretching the blood vessel. While such balloon angioplasty procedures have proven to be an efficacious and relatively safe means of treating some vaso-occlusive lesions, such balloon dilation procedures have proven less than optimal in the treatment of certain advanced lesions such as those consisting of fibrous or calcified atherosclerotic plaque. Moreover, by its very nature, the balloon angioplasty technique does not remove the occlusive lesion but, rather, simply compresses the lesion and/or dilates or stretches the surrounding blood vessel wall. There is a propensity for at least some balloon-treated lesions to subsequently re-occlude some time after the balloon angioplasty procedure has been carried out. In some cases, full or partial re-occlusion of the blood vessel may result, thereby necessitating a repeat of the angioplasty procedure or other additional means of intervention (e.g. coronary artery bypass surgery).
Thus, it remains desirable to develop new alternatives to balloon dilation angioplasty procedures whereby the offending plaque or other stenotic lesion is actually removed, rather than merely stretched or compressed.
One viable alternative to the balloon dilation angioplasty technique is "ultrasonic" angioplasty. A number of methods and apparatus for ultrasonic angioplasty have been described in the prior art. One type of ultrasonic angioplasty apparatus known in the prior art comprises a flexible intravascular catheter having an ultrasound transmission wire (syn. "ultrasound transmission member") which extends longitudinally therethrough. The distal end of the ultrasound transmission wire is advanceable out of the distal end of the catheter. A bulbous distal head or probe is formed or mounted on the distal end of the ultrasound transmission wire. When ultrasonic energy is passed through the transmission wire, the distal head or probe will undergo vibratory movement in accordance with the sonic energy being transmitted therethrough. Such vibratory movement of the distal head or probe causes disintegration and ablation of the adjacent lesion, without damage to or perforation of the surrounding blood vessel wall.
Examples of ultrasonic angioplasty devices and related apparatus of the prior art include those described in U.S. Pat. Nos. 3,433,226 (Boyd), 3,823,717 (Pohlman et al.), 4,808,153 (Parisi), 4,936,281 (Stasz), 3,565,062 (Kuris), 4,924,863 (Sterzer), 4,870,953 (DonMichael et al.); and other Patent Publications WO87-05793 (Cooper), WO89-06515 (Bernstein et al.), WO90-0130, (Sonic Needle Corp.); EP 316 796 (DonMichael, et al.); DE 3,812,836 (Schubert) and DT 2,438,648 (Pohlman).
In particular, one prior art ultrasonic angioplasty device disclosed in U.S. Pat. No. 4,870,953 and European patent publication EP 316 796 (DonMichael), is purported to effectively remove advanced fibrous or calcified atherosclerotic lesions as well as other thrombotic vascular occlusions. The device of U.S. Pat. No. 4,870,953 (DonMichael) comprises a flexible catheter having a wire like elongate ultrasound transmission member which extends longitudinally through the catheter. A bulbous distal head or probe is formed on the distal end of the ultrasound transmission member. The bulbous distal head may be alternately advanced out of and retracted into the distal end of the catheter. An ultrasound generation device is connected to the proximal end of the transmission member. When the ultrasound generation device is actuated, sound waves are passed through the transmission member, causing the bulbous distal head to undergo transverse and forward/backward vibration. Such vibratory movement of the bulbous distal head allows the distal head, when positioned adjacent an atherosclerotic lesion, to break up and dissolve such atherosclerotic material in two (2) ways. First, it is purported that such vibratory movement of the distal head will effect direct "chipping" or "beating" of the occlusive material. Second, such vibratory movement of the distal head is purported to create turbulence or cavitation at the treatment site, such that said turbulence or cavitation will coact with and enhance the direct "chipping" or "beating" action of the distal head.
The entire disclosure and drawings of U.S. Pat. No. 4,870,953 (DonMichael, et al.) is expressly incorporated herein by reference.
One area in which the device of U.S. Pat. No. 4,870,953 (DonMichael et al.) and/or other prior art ultrasonic angioplasty devices may be subject to improvement is by improving the structural, physical and acoustical properties of the ultrasound transmitting member or wire which serves to transmit the sonic energy through the length of the catheter. While it remains desirable that the ultrasound transmission member be endowed with sufficient flexibility and elasticity to navigate sharp bends and curves in the vascular anatomy, it is also desirable that the ultrasound transmission member or wire be of sufficient mass or thickness to transmit of the required amount of sonic energy therethrough. Additionally, it is desirable that the ultrasound transmission member or wire be sufficiently strong to prevent untoward breakage or fracture of the wire during use.
A second area in which the prior art ultrasonic angioplasty devices may be subject to improvement is in the configuration and design of the distal head and/or other portions of the ultrasound transmission member so as to optimize the therapeutic efficacy of the device in fragmenting and removing occlusive material from the surrounding artery walls.
ii. Superelastic Materials
As will be described fully herein, one embodiment of the present invention incorporates and utilizes an ultrasound transmission member or wire formed of one or more material(s) which exhibit what is known as "superelasticity". The term "superelasticity", as used herein, shall generally mean the ability of a material to undergo large elastic deformations without the onset of plasticity or permanent deformation.
One particular group of metallic alloys, known as "shape memory alloys" is known to exhibit superelastic properties. Such shape memory alloys also demonstrate a "shape memory" phenomenon whereby the alloy is generally transformable, back and forth, between a low temperature configuration and a high temperature configuration. Transformation from the low temperature configuration to the high temperature configuration generally occurs as the alloy passes from its martensitic state to its austenitic state. Subsequent transformation from the high temperature configuration back to the low temperature configuration occurs as the alloy passes from its austenitic state to its martensitic state.
In addition to their unique ability to undergo temperature controlled changes in configuration, the shape memory alloys also exhibit temperature-dependent superelastic properties. The superelastic properties of the shape memory alloys are typically exhibited while the alloys remain in their martensitic states, but are lost as the alloys transition from their martensitic states to their austenitic states. Also, the degree or amount of superelasticity exhibited by a given shape memory alloy is generally dependent upon the stoichiometric ratio of the individual metal elements within that particular alloy.
Although one prior inventor has recognized that certain titanium alloys may exhibit better stress strain characteristics than those exhibited by high strength stainless steel or certain other metal alloys previously used for the transmission of sonic energy in ultrasound ablation devices (U.S. Pat. No. 4,920,954 and WO90-01300 (Alger et al.), the prior art has heretofore failed to identify specific titanium or other metal alloys which provide the most suitable combination of a) strength, b) flexibility/elasticity and c) sound transmission properties for use in ultrasound ablation devices.
Accordingly, there remains a need in the art for the development of improved ultrasonic angioplasty devices, having ultrasound transmitting wires and/or ablation probes which are configurationally and/or materially modified to exhibit an improved combination of strength, flexibility/elasticity and sound transmitting characteristics, within the usual range of operating temperatures encountered in the ultrasonic angioplasty device.